1. Field of the Invention
The present invention relates to a motor.
2. Description of the Related Art
In the past, a technique has been known in which a sensor magnet is mounted on a rotating section of a motor and then the position of the sensor magnet is magnetically detected. In this way, the rotational position or the number of rotations of the rotating section of the motor can be detected. For example, in Japanese Unexamined Patent Application Publication No. 2008-219996, a motor including a sensor magnet and a Hall element is described. The sensor magnet is fixed to a shaft through a yoke. The Hall element faces the sensor magnet. In the motor described in the above publication, the magnetic pole of the sensor magnet is detected by the Hall element. In this way, an angular position around a central axis of a rotor core with respect to an armature is detected.
In the motor described in Japanese Unexamined Patent Application Publication No. 2008-219996, the Hall element is disposed on the lower surface of a circuit board above the sensor magnet. Further, a back yoke, which is a magnetic body, is fixed to the upper surface of the circuit board. Then, magnetic flux density toward the Hall element from the sensor magnet is increased by the back yoke. In this way, detection accuracy of a sensor is improved (refer to Paragraph [0059] of Japanese Unexamined Patent Application Publication No. 2008-219996).
However, if such a back yoke is fixed to the circuit board by a plurality of screws, there are cases where the back yoke will be bent by the tightening force of the screws. If the back yoke is bent, an undesirable clearance will be produced between the upper surface of the circuit board and the lower surface of the back yoke. This clearance makes it difficult to sufficiently improve the detection accuracy of the sensor.
In addition, a lowering of an amount of detection accuracy caused by the flexure of such a magnetic member can be within an acceptable range depending on the application of the motor. However, in order to provide a higher quality motor, it is required to improve detection accuracy of a magnetic sensor by suppressing the flexure of the magnetic member.